Sleep deprivation is inevitable in the military environment where the battlefield situation often involves 24 hours or more of continuous operation. Maintaining a high level of alertness and cognitive performance under demands of constant readiness around the clock is not only individually difficult, but also impossible to assess without a means of direct monitoring. The U.S. Army has long been concerned about the potential for catastrophic outcomes as consequences of sleep deprivation which result in poor judgment and performance on the part of military personnel whose decision making ability is impaired. This concern has been realized in the immediate past with the capture of soldiers in Iraq who took a wrong turn on a road and found themselves in unknown enemy territory It was later revealed that the soldiers on this expedition had been without sleep for longer than 24 hours, were cognitively impaired and unaware that they had misread the map. Boot, M., The new American way of war, Foreign Affairs, July/August 2003.
Consequential incidents due to sleep deprivation and sleep restriction pertain not only to the military, but also to the public sector in areas of transportation, nuclear facilities, emergency support, and health care providers among the more immediate concerns. Many incidents or near accidents occurring in the public arena are not publicized especially those involving pilot fatigue. Congress is currently investigating why these incidents are not reported to the public and cites the case on Mar. 4, 2004, where both pilot and co-pilot flew three sequential “red eyes” between Denver and Baltimore with only one hour in between flights. During the last 45 minutes of the third flight as it was approaching Denver, both pilot and co-pilot were sound asleep and missed all calls from the air traffic controller while the plane was traveling at 590 mph instead of less than 290 mph. Fortunately, the pilot did suddenly awake to hear the air traffic controller's frantic calls and was able to follow his instructions resulting in a safe landing. Foxnews.com, Pilot 1st Officer Slept While Approaching Denver, Lawmaker Says, Oct. 31, 2007.
Even more alarming are the results from a 1992 survey of tractor trailer truckers which found that 19% of the truckers admitted to having fallen asleep at the wheel in the previous month. Braver, E R et al., Long hours and fatigue: a survey of tractor-trailer drivers, Journal of Public Health Policy, 1992, Vol. 13, No. 3, pp. 341-366. A report from the Center for National Truck Statistics in 1994 included the disturbing statistic that annually over 5,000 fatalities and 110,000 injuries resulting from motor vehicle accidents involve commercial trucks in the United States. Center for National Truck Statistics, Truck and bus accident factbook 1994, Federal Highway Administration Office of Motor Carriers, 1996 (Report no. UMTRI-96-40). Knipling estimated that the percentage of vehicle crashes in which fatigue was a factor could be as high as 56%. Knipling, R R et al., Crashes and fatalities related to driver drowsiness/fatigue: research note, National Highway Traffic Safety Administration, 1994. Although the Department of Transportation (DOT) regulates work hours permitted for truck drivers, pilots, airport controllers and railroad engineers, there is no routine checking for status of alertness (hence being well rested) just prior to start of duty or during duty hours.
A more recent example documented security guards in a Pennsylvania nuclear plant were regularly asleep on the job for periods exceeding one hour. This public exposure resulted in loss of employment and dismissal of the security company providing the staff, but offered no remedy as to how this could be prevented in the future. (Weinberger, 2007).
Brain electrical activity, commonly referred to as electroencephalogram (EEG), is the manifestation of neuronal communication which may be discerned and recorded at the surface of the scalp by electrode sensors and subsequently displayed, measured, and analyzed. Clinically, the EEG is used for detection of brain pathology such as tumors, epileptic seizures, and behavioral abnormalities such as narcolepsy and attention deficit hyperactive disorder (ADHD). The brain signals, collectively referred to as an electroencephalograph, are analyzed for their constituent frequencies (rhythmic oscillations) and/or selective characteristic wave shapes to detect deviations from normal. In the sleep research laboratory, the EEG is used not only for determination of sleep/wake states and for quantification of sleep amount during nighttime sleep but also to track sleepiness level during the course of sleep deprivation studies of normal, healthy individuals.
Polysomnography is the methodology for defining the awake and sleep states from observation of EEG signals over an extended time period. As its name implies, other physiological measures are recorded synchronously with the EEG to aid in differentiating the awake from the sleep state as well as marking the different stages of sleep. Multiple electrode sensors are attached to the scalp, face, and body of the individual under study to record both the neurophysiological (EEG) and basic physiological measures such as electrooculogram (EOG) for recording eye movements; submental electromyogram (EMG) from the chin for detecting muscle movement; and electrocardiogram (EKG) for heart rate. Although the EEG is the main determinant of sleep characteristics, the EOG and EMG aid in defining Rapid Eye Movement (REM) sleep more commonly known as the dream stage in which it is conjectured that memory consolidation occurs. REM is thus distinguished from non-Rapid Eye Movement (NREM) sleep which defines all other sleep stages. Rolling eye movements observed in the EOG are characteristic during REM simultaneously with muscle atonia as noted in the EMG. During night time sleep, the REM state alternates with non-REM sleep in ultradian cycles of approximately 90 minutes and increases in length as non-REM length decreases in the progression towards the end of the sleep period. The EKG provides continuous monitoring of heart rate not only to assure normal functioning, but also to confirm the deeper sleep stages when the reduced rate of heart beats indicates slowing of body functions.
Although the frequency realm of EEG is in cycles per second or Hertz (Hz) and several orders of magnitude higher than that of ultradian frequencies (i.e., cycles/24 hours), the same fundamental principles of rhythmic behavior apply. The EEG signal as visually observed in its entirety is a combination of all the frequencies selected for recording in the acquisition process. Overall circadian rhythmicity is observed in the oscillation of the frequencies and depending on the frequency band, the cyclic variation mimics the circadian or is out of phase by 180°. That is, the band of low frequencies peaks in the hours of sleep while the band of high frequencies peaks during the waking active period.
Sleep researchers have devised an EEG (or polysomnography (PSG)) scoring system, considered to be the “gold standard” for evaluating sleep depth according to specific frequencies and patterns of EEG waveforms as established by Rechtschaffen and Kales. Rechtschaffen, A. et al., A manual of standardized terminology, techniques and scoring system for sleep stages of human subjects, Public Health Service, U.S. Government Printing Office, 1968 (reprinted 1971). The system consists of 6 levels in which sleep is scored within a 20 second or 30 second standard epoch as: Wake; Stage 1; Stage 2; Stage 3; Stage 4; and REM. By conventional Rechtschaffen and Kales practice, the EEG during the awake state consists mainly of frequencies between 12-50 Hz and is known as beta frequency although it is sometimes subcategorized as gamma in the 30-40 Hz range. Stage 1 is considered light sleep and the EEG is defined by a mix of predominantly alpha (7-14 Hz) and some theta (5-7 Hz) frequencies. It is not difficult to be awakened at this stage. Stage 2 is deeper sleep dominated by theta, along with some alpha. This stage is characterized by intrusions of specific wave patterns described as K-complexes and spindles because they resemble these descriptions and appears to be the threshold to actual sleep whereas Stage 1 is more the transitional state between awake and sleep. Stages 3 and 4 are marked by delta (1-4 Hz) frequencies with Stage 3 showing all of these frequencies while Stage 4 have both greater percentage and higher amplitude of 1 Hz and 2 Hz frequencies. Stage 4 represents the deepest sleep stage where frequency of neuronal communication is lowest and judging from the high amplitude of these lowest frequencies, indicative that the sum of active brain function is essentially minimal throughout the brain, i.e., the brain has essentially “shut down”. Arousal from this sleep stage is extremely difficult.
Sleep scorers mark latency to sleep with the appearance of K-complexes or sleep spindles seen in Stage 1 or 2. Latency of 5 minutes or less is considered pathological in the clinical setting under normal conditions, but in the sleep research laboratory, this is quite often the case in sleep restricted or sleep deprived individuals with no existing pathology.
It is to be noted that EEG records are usually visually scanned and manually Scored—a long, tedious process with emphasis of the process on either the sleep or awake state and little or no attention directed to the between state of drowsiness. There has been little change in manual EEG sleep stage scoring for over 35 years, until recently where attempts have been made to automate the procedure with some measure of accuracy by following the Rechtschaffen and Kales guidelines as well as the more recent American Association of Sleep Medicine's The AASM Manual for the Scoring of Sleep and Associated Events. Anderer, P., An E-health solution for automatic sleep classification according to Rechtschaffen and Kales validation study of the Sommolyzer 24×7 utilizing the Siesta database, Neuropsychobiology, 2005, Vol. 51, No. 3, pp. 115-123.
Most commercial EEG systems are designed to record up to about 256 Hz, because that is the upper limit for extracting useful information in PSG scoring. As a result, there has been no need to examine EEG data for the frequencies above 256 Hz.
There is a general emphasis in making polysomnographic determinations of whether a person is either sleep or awake with little attention directed to the in between states of drowsiness or alertness. Existing alertness systems are looking for physical manifestations indicating that a person is alert or not alert. Methods and apparatuses related to alertness detection fall into five basic categories: a method/apparatus for unobtrusively monitoring current alertness level; a method/apparatus for unobtrusively monitoring current alertness level and providing a warning/alarm to the user of decreased alertness and/or to increase user's alertness level; a method/apparatus for monitoring current alertness level based on the user's responses to some secondary task possibly with an alarm device to warn the user of decreased alertness and/or to increase user's alertness level; methods to increase alertness; and a method/apparatus for predicting past, current, or future alertness.
These methods and apparatuses that unobtrusively monitor the current alertness level are based on an “embedded measures” approach. That is, such methods infer alertness/drowsiness from the current level of some factor (e.g., eye position or closure) assumed to correlate with alertness/drowsiness. Issued patents of this type include U.S. Pat. No. 5,689,241 to J. Clarke, Sr., et al. disclosing an apparatus to detect eye closure and ambient temperature around the nose and mouth; U.S. Pat. No. 5,682,144 to K. Mannik disclosing an apparatus to detect eye closure; and U.S. Pat. No. 5,570,698 to C. Liang et al. disclosing an apparatus to monitor eye localization and motion to detect sleepiness. An obvious disadvantage of these types of methods and apparatuses is that the measures are likely detecting sleep onset itself rather than small decreases in alertness.
In some patents, methods for embedded monitoring of alertness/drowsiness are combined with additional methods for signaling the user of decreased alertness and/or increasing alertness. Issued patents of this type include U.S. Pat. No. 5,691,693 to P. Kithil describing a device that senses a vehicle operator's head position and motion to compare current data to profiles of “normal” head motion and “impaired” head motion. Warning devices are activated when head motion deviates from the “normal” in some predetermined way. U.S. Pat. No. 5,585,785 to R. Gwin et al. describes an apparatus and a method for measuring total handgrip pressure on a steering wheel such that an alarm is sounded when the grip pressure falls below a predetermined “lower limit” indicating drowsiness. U.S. Pat. No. 5,568,127 to H. Bang describes a device for detecting drowsiness as indicated by the user's chin contacting an alarm device, which then produces a tactile and auditory warning. U.S. Pat. No. 5,566,067 to J. Hobson et al. describes a method and an apparatus to detect eyelid movements. A change in detected eyelid movements from a predetermined threshold causes an output signal/alarm (preferably auditory). As with the first category of methods and apparatuses, a disadvantage here is that the measures are likely detecting sleep onset itself rather than small decreases in alertness.
Other alertness/drowsiness monitoring devices have been developed based on a “primary/secondary task” approach. For example, U.S. Pat. No. 5,595,488 to E. Gozlan et al. describes an apparatus and a method for presenting auditory, visual, or tactile stimuli to an individual to which the individual must respond (secondary task) while performing the primary task of interest (e.g., driving). Responses on the secondary task are compared to baseline “alert” levels for responding. U.S. Pat. No. 5,259,390 to A. MacLean describes a device in which the user responds to a relatively innocuous vibrating stimulus. The speed to respond to the stimulus is used as a measure of the alertness level. A disadvantage here is that the apparatus requires responses to a secondary task to infer alertness, thereby altering and possibly interfering with the primary task.
Other methods exist solely for increasing alertness and depend upon the user to self-evaluate alertness level and activate the device when the user feels drowsy. An example of the latter is U.S. Pat. No. 5,647,633 and related patents to M. Fukuoka in which a method/apparatus is described for causing the user's seat to vibrate when the user detects drowsiness. Obvious disadvantages of such devices are that the user must be able to accurately self-assess his/her current level of alertness, and that the user must be able to correctly act upon this assessment.
Methods also exist to predict alertness level based on user inputs known empirically to modify alertness. U.S. Pat. No. 5,433,223 to M. Moore-Ede et al. describes a method for predicting the likely alertness level of an individual at a specific point in time (past, current or future) based upon a mathematical computation of a variety of factors (referred to as “real-world” factors) that bear some relationship to alterations in alertness. The individual's Baseline Alertness Curve (BAC) is first determined based on five inputs and represents the optimal alertness curve displayed in a stable environment. Next, the BAC is modified by alertness modifying stimuli to arrive at a Modified Baseline Alertness Curve. Thus, the method is a means for predicting an individual's alertness level, not cognitive performance.
More recently a method was developed that uses information in an EEG signal in frequency bands above 30 Hz, for example, 80-420 Hz. U.S. Pat. No. 5,813,993 to Kaplan et al. describes such a method that uses a weighted sum of the inverse of the energy of a subject's EEG signal in selected frequency bands. The energy level for each frequency band is inverted and then weighted prior to the inverted energy levels being summed together to provide a score reflective of the subject's alertness or drowsiness.